This disclosure relates generally to communication devices, and more particularly, to communication headsets that utilize a movable microphone boom.
Communications headsets can be used in a wide range of applications, and are particularly effective for use with mobile communications devices, such as cellular telephones. The use of communications headsets with mobile communications devices depends heavily on the headsets' ability to provide consistently high transmit signal quality, of which one important measure is the signal-to-noise ratio (i.e., the ratio between the level of the signals associated with the desired acoustic source, such as the user's voice, and those from ambient noise). Hence, it is desirable for communications headsets to include mechanisms that can provide a high signal-to-noise ratio when the headsets are used in noisy environments. It is particularly advantageous to be able to reduce the obscuring effect of ambient noise in the transmit signals when the headsets are used outdoors.
Previous noise reduction designs often involve complicated, expensive electrical circuitry, which are both delicate and prone to errors. For example, noise-canceling microphones, i.e., microphones that are more sensitive to sound waves approaching in certain directions relative to the others, have been designed for use in noisy environments. These microphones are constructed such that both sides of their diaphragms are exposed to sound waves, and reduce the noise content of the transmit signals, thereby increasing the signal-to-noise ratio. Another class of solutions involves the use of sophisticated signal processing techniques to reduce the level of noise content in the transmit signals. Both types of solutions have certain deficiencies.
Noise-canceling microphones are typically placed at the end of a long boom so that they can be aimed toward the user's mouth. When used in a noisy environment, a noise-canceling microphone can increase the signal-to-noise ratio of the output signals for two reasons. First, provided the microphone is aimed toward the user's mouth, sound waves from the user's voice approach the microphone in or near its direction of maximum sensitivity. The ambient noise, on the other hand, is usually diffuse, and approaches from many different directions. Thus, only a small portion of the noise approaches the microphone in the direction of its highest sensitivity. Even if the noise source is non-diffuse, i.e., the noise originates from one or a few specific directions, there is a high probability that a large portion of the noise approaches from directions in which the microphone is relatively insensitive.
The second reason for the increase in signal-to-noise ratio relates to a phenomenon known as the “proximity effect.” In essence, the proximity effect relates to directional microphones responding strongly to sound waves from nearby sources. This is because, by virtue of the curvature of the wave fronts of sound originated from a small, nearby source, the amplitude difference between the arrivals of the waves to the front and to the rear of the microphone's diaphragm becomes significant, particularly at low frequencies. The noise-canceling microphone is therefore more sensitive to the user's voice than to the ambient noise from more distant sources.
The advantages of a noise-canceling microphone can be realized only if the acoustic sensing point is disposed close to the user's mouth and appropriately positioned (i.e., in front of, rather than behind, the cheekbone). To satisfy these conditions generally requires a sufficiently long boom that provides the user with enough flexibility to direct or aim the microphone toward his or her mouth. In addition, the improved performance of a noise-canceling microphone depends largely on the assumption that the ambient noise is diffuse or that it approaches from an angle outside the microphone's angular range of maximum sensitivity, which is not always the case. Moreover, noise-canceling microphones are known to be more susceptible to wind noise than omnidirectional microphones because of the pressure turbulence resulting from wind blowing on the microphone. In fact, as the directivity factor of a noise-canceling microphone increases, so does the ratio of wind noise sensitivity to voice sensitivity.
Long booms, which place the acoustic sensing point near the user's mouth, as required for effective noise canceling, are not always desirable in communications headsets. Headsets with short booms or no booms at all are sometimes appealing because of their unobtrusiveness, more stylish appearance and easy stowability. This is particularly the case with users of portable communication applications, such as mobile phones. It is therefore desirable that communications headsets be designed with multiple modes of operation, including at least a mode featuring a long boom extending near the user's mouth to communicate in noisy environments, and a compact mode that provides convenience when ambient noise is not a problem.
There are other methods that employ signal processing techniques to reduce the undesirable effect of ambient noise in the transmit signals. One such technique is voice expansion, which is a form of dynamic signal processing that dynamically adjusts the amplification gain (i.e., the ratio between the levels of the amplified output signals and the raw electrical signals as converted by the microphone from acoustic signals before the amplification takes place) as a function of the transmit signal level. Hence, when a communications headset equipped with a voice expansion mechanism is used in a noisy environment, voice expansion serves to reduce the level of output signals, including both the signals originating from the desired source and the ambient noise when the signal level is low.
A related problem with conventional communications headsets that operate in multiple modes with different boom lengths is that of sound distortion in the audio transmission. The limited dynamic range of telephone lines or wireless channels may result in distortion in the transmission. In addition, noise-canceling microphones have different frequency response curves for acoustic sources located at different distances due to the proximity effect above. Further, even if an omnidirectional microphone is used, there may still be a shift in the sound spectrum associated with any change in the location of the acoustic sensing point relative to the user's mouth. This is because high frequencies are attenuated more than low frequencies when the sound travels through air.
Accordingly, it is desirable to provide a communications headset using different boom lengths to operates in multiple modes with a mechanism that automatically adjusts the headset for optimal voice sensitivity in each mode, and that is simple and inexpensive to make and use.